1. Field of the Invention
The present invention relates generally to a tunable optical filter, and more particularly to a tunable optical filter having a variable stop band.
2. Description of the Related Art
As a technique for constructing a flexible optical fiber network, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The optical carriers are individually modulated to obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is launched into an optical fiber transmission line. At a receiving end, the WDM signal light received is separated into a plurality of individual optical signals by an optical demultiplexer to reproduce transmitted data according to each optical signal.
In such a system adopting WDM, an add/drop function is important. The add function is a function of joining an optical signal of a specific wavelength channel into WDM signal light, and the drop function is a function of extracting an optical signal of a specific wavelength channel from WDM signal light.
As an optical device for effecting the add/drop function, a tunable optical filter is known. In the case that the tunable optical filter is a bandpass filter, its pass band is variable, whereas in the case that the tunable optical filter is a notch filter, its stop band is variable.
Referring to FIGS. 1A and 1B, there are respectively shown the configuration and operating characteristic of a conventional tunable optical filter having a variable pass band. As shown in FIG. 1A, this tunable optical filter includes a first port 2, an optical filter member 4 having a characteristic changing according to a position in a predetermined direction (e.g., a direction parallel to the plane of the sheet of FIG. 1A), a second port 6 optically coupled to the first port 2 by a transmitted light path concerning the optical filter member 4, and a mechanism 8 for displacing the optical filter member 4 in the above predetermined direction.
The optical filter member 4 may be configured by changing the thickness of a multilayer dielectric film in the predetermined direction. When the optical filter member 4 is displaced by the mechanism 8, the pass band is shifted along a wavelength axis as shown in FIG. 1B. In FIG. 1B, the vertical axis represents transmittance (T) between the ports 2 and 6, and the horizontal axis represents wavelength (.lambda.).
Referring to FIGS. 2A and 2B, there are respectively shown the configuration and operating characteristic of a conventional tunable optical filter having a variable stop band. This tunable optical filter includes a first port 10, an optical filter member 12 having a characteristic changing according to a position in a predetermined direction (e.g., a direction parallel to the plane of the sheet of FIG. 2A), a second port 14 optically coupled to the first port 10 by a transmitted light path concerning the optical filter member 12, a third port 16 optically coupled to the first port 10 by a reflected light path concerning the optical filter member 12, and a mechanism 18 for displacing the optical filter member 12 in the above predetermined direction.
In the case that the optical filter member 12 has a characteristic similar to that of the optical filter member 4 shown in FIG. 1A, the wavelength characteristic of transmittance between the first port 10 and the second port 14 is variable along a wavelength axis as in FIG. 1B. Further, as shown in FIG. 2B, the wavelength characteristic of reflectance between the first port 10 and the third port 16 is variable. In FIG. 2B, the vertical axis represents reflectance (R), and the horizontal axis represents wavelength (.lambda.). The characteristic shown in FIG. 2B is reverse to the characteristic shown in FIG. 1B.
In the optical coupling by the transmitted light path as shown in FIG. 1A, the transmittance is almost stable irrespective of an angular deviation of the optical filter member 4 due to play or the like in the mechanism 8. In the case that the transmitted light path is provided by a parallel beam, for example, shifting of the parallel beam due to the angular deviation has almost no effect on a coupling efficiency between the ports 2 and 6.
However, in the case that the first port 10 and the third port 16 are optically coupled by the reflected light path as shown in FIG. 2A, an angular deviation of the optical filter member 12 has a direct effect on the reflectance, causing instability of the operation. In the case that the reflected light path is provided by a parallel beam, for example, the angular deviation has a profound effect on a coupling efficiency between the ports 10 and 16. Thus, the prior art tunable optical filter having a variable stop band has a problem that the operation becomes unstable.